This invention relates to the surface character of a thin film magnetic disk, and more particularly to a magnetic disk having a preferable surface character with respect to a durability of the disk surface such as head flyability, contact start-stops characteristic and head stickiness for a thin film magnetic disk.
In the conventional system, it has been found that as described in Japanese Patent Application Laid-Open No. 62-219227 a substrate having micro projections in a substrate for a thin film magnetic disk (a surface work for forming the micro projections is hereinafter referred to as texture) has surface unevenness with its maximum roughness of 0.02 to 0.1 .mu.m, and as shown in FIG. 10, a non-magnetic metallic layer 31 and a magnetic thin film medium 32 are formed on the substrate 1, resulting in that a coercive force of the thin magnetic film becomes more than 500 Oe in the above-mentioned maximum surface roughness and so a non-magnetic metallic film (a chromium film) can be made thin to improve productivity. As a result of a contact start-stop test, no scars have been found on the disk surface under twenty thousand times of testing. However, when the maximum surface roughness is more than 0.1 .mu.m, a head crush may easily occur and if no texture is applied, a scar may occur at times over 5,000 times of a contact start-stop test and then the head crush is generated.
As the conventional type of texture, as an example, as described in Jap. Pat. Laid-Open No. 54-23294, it employs a method and a disk working device as illustrated in FIGS. 7, 8 and 12. In the drawings, 2 designates a substrate relating to a disk, central lines of a pair of contact rollers 8 (both of them are made of resilient rubber) are arranged in a radial direction of this disk so as to hold this disk and then a grinding tape 4 (4A and 4B) running upward and downward is placed between the contact roller 8 (8A and 8B) and the disk 2. Simultaneously with a pressing of the contact roller and a rotation of the disk, the contact roller 8 is reciprocated and slid in a radial direction, to make a simultaneous working of both surfaces of the disk. According to this texture, it is possible to form micro projections without grinding unevenness. However, it shows a problem that a non-stable raising part is generated at the shoulder portions of the micro projections along with this formation and the raised portions are left as micro projections on the surface.
It is described that in the prior art, uneven surfaces with a maximum surface roughness of 0.02 .mu.m to 0.1 .mu.m are formed on the surface of the substrate (texture), thereby even if a thickness of the non-magnetic metallic layer (chromium film) is made thin, a coercive force of the magnetic thin film medium (Co--Ni) is more than 500 Oe and it may fulfill a contact start-stop characteristic of twenty thousand times without generating any head crush. However, in the prior art, only the maximum surface roughness is restricted or defined, and so if a texture for forming micro projections is applied to the substrate covered with aluminum alloy or anode oxidization aluminum or Ni--P plating or the like, fine projections are generated on the shoulder portions of the micro projections. Due to the fine projections, if a floating test for the head is carried out in a narrow clearance, for example, in a head floating clearance of 0.2 .mu.m, the head and the micro projections may intermittently contact each other, resulting in damaging a head floating characteristic to cause a head crush. In addition, the upper-most surface of the disk is varied under a sliding movement of the head in case of performing a test of contact start-stops, a disk surface is made smooth under this variation, a horizontal resistance applied to the head is increased and this causes head crush. In view of this fact, it is important to define the micro projections under the texture and a mere definition of the maximum surface roughness can not explain the effects on disk start-stops or head crush. No consideration is made of a shape of projection or a repetition time from the mean surface rather than from the maximum surface roughness. In addition, no most important proposal for the shape of the uneven surface of the substrate has been identified for improving the head crush or a start-stops characteristic.
In this specification, the micro projection designates a respective mountain having a minute height and located in a pseudo circumferential direction of the disk or in respect to the texture formed helically as shown "A" in FIG. 31, as to the surface roughness curve being measured in a radial direction of the disk from its central line to the projected direction. A height of the micro projection designates a distance between a top of a respective mountain and the central line. In FIG. 31, a reference character B designates a curve of section and a reference character C designates a center line, respectively.